As the thermal transfer recording technology, there is a method in which a recording material (hereinafter occasionally referred to as an ink sheet) comprising a base material having thereon a thermally fusible ink layer or an ink layer comprising a thermally sublimable dye, and an image-receiving material (occasionally referred to as an image-receiving sheet) are opposed each other, and a heat source controlled by electric signals of a thermal head, an electricity-turning head, etc. is provided from the side of the ink sheet pressed for contact, and images are thus transfer-recorded. The thermal transfer recording provides advantages such as silence, very negligible requirements for maintenance, low cost, easy formation of color images, availability of digital recording and the like. Accordingly, the thermal transfer recording has been employed in a variety of fields such as printers, recorders, facsimile machines, computer terminals and the like.
The sublimable type thermal transfer recording has received attention because of its advantages in the adaptation to color image formation and tone reproduction. Further the quality of formed images is comparable to that obtained by the photographic method employing silver salts. However, the images formed by the sublimable dye has been noted to exhibit a problem of fixability or immobilization.
In order to improve the fixability, a method has been known in which after completing the image formation, the image further undergoes thermal treatment to yield prescribed dye formation, while pushing the dye into the interior of the image-receiving layer. For example, Japanese Patent Publication discloses that a non-transfer region, where no sublimable dyes is coated, is provided or the successive face order sublimable transfer sheet, and during thermal transfer printing, the non-transfer sheet completing the transfer is heated again. However, this method requires a large useless area on the transfer sheet and causes an increase in material waste.
Furthermore, for improving the stability of images as well as solving the fixability problem, a method has been proposed in which an image-protecting layer is formed. Methods for forming such an image-protecting layer are: lamination, transfer of transfer foil on the image, etc. However, these methods need a non-transfer support and a laminating material which are durable under high temperatures, causing an increase in cost.
As a method to obtain sufficient image stability without employing an image-protecting layer, it has been proposed that a thermally diffusible chelatable dye (hereinafter referred to as a post-chelate dye) is transferred into an image-receiving layer comprising it compound containing metal ions to improve the fixability through the chelation in the image-receiving layer. Japanese Patent Publication Open to Public Inspection Nos. 59-78893, 59-109349, 60-2398, etc., for example, describe such a method. In these patents, it is illustrated that the light fastness and fixability of the images formed, which employ the post-chelate dye, are much improved compared to those formed by employing conventional sublimable dyes.
The post-chelate dyes, disclosed in the above-mentioned patents, etc., are those which form bidentate-ligand or tridentate-ligand chelate dyes. In the thermal transfer recording employing these dyes, there is a large difference in hue between the post-chelate dye and the chelate dye. As a result, when the chelation reaction is not sufficiently completed, the color reproduction area is decreased due to the occurrence of undesirable secondary absorption and have occasionally caused insufficient image stability. Due to these, it is proposed to carry out a post thermal treatment and the like to fully complete the chelation reaction.
In order to complete the chelation reaction, the image materials are subjected to high temperatures in the range of about 150 to about 200.degree. C. Accordingly, as heating devices, thermal heads and heat rollers are employed which save space. The heat rollers can be preferably employed as economical materials. When the image-receiving material completing the image formation is processed employing a heat roller device, not only is the roller strained with the transfer of the chelate dye, but also when another image-receiving material completing the other image formation is processed, the dye transferred to the roller is again transferred back to the surface of the image-receiving material which markedly degrades the image quality due to such stains. Furthermore, direct-heating the surface of the image employing the thermal head damages the image due to the thermal head.
Furthermore, as technology in regard to the post-heating processes, Japanese Patent Publication No. 4-55870 discloses a technology in which the post-heating is carried out, via the part of an ink sheet where no dye is coated, employing the same thermal head as that utilized to form the image. Japanese Patent Publication Open to Public Inspection No. 7-108772 discloses a technology in which, after forming an image employing a thermal head, the surface of the image is subjected to heating process via a sheet of plastic film employing a thermal head that is different from that employed to form the image. In these technologies, are mainly employed the film which is connected with an ink sleet in the face order and in the same way as for the ink sheet, one sheet is consumed for one image. And these films employed for post-heating do not particularly perform any processing and when brought into contact with the surface of the image, the dye is reversibly transferred; when the same film is repeatedly employed, it occasionally adheres onto the surface of the next image. Thus, these cause problems such that material waste increases together with an increase in ink sheets and image density decreases due to the reverse transfer of the dye to the film.